[FieldValueSelectionFilter] Migrate from class to function component

`FieldValueSelectionFilter` renders a popover with a selectable list for field value filtering within `EuiSearchBar`. Internal component, not exported.

- **File:** `components/search_bar/filters/field_value_selection_filter.tsx`
- **Class size:** 342 lines
- **State:** Constructor-initialized (options, query, active state)
- **Lifecycle:** `componentDidMount`, `componentDidUpdate`, `componentWillUnmount`

**Migration notes:**

Full lifecycle suite managing async option loading and cache. The most complex of the SearchBar filter components.

**Acceptance criteria:**

- [ ] Convert to function component with hooks
- [ ] Maintain async option loading
- [ ] All existing tests pass